In the discussions in the comments of this answer, this jaw dropping point was mentioned:
It's aus denen. There's a preposition in front so it can't be the subject. Same for auf dieser Erde.
Is this always true? I have a hard time believing this as in English we can actually have prepositional object as subject see here. And, since German is very similar to English in a first order approximation, it seems very weird.
So this is all about the English sentence
From five to seven would be the best time.
from your second link, and its literal German equivalent
Von fünf bis sieben wäre die beste Zeit.
which is indeed valid German. But I think the author of that article is confused about the English, and especially the word order in her own example. That's a copula phrase and that means both items right and left of the verb refer to the subject.
In that special corner case, English lifts its strict word order rule. The sentences
The best place is there.
and
There is the best place.
are equivalent apart from the very fact that the latter is topicalized. It's not obvious because English topicalized sentences usually start with the topic separated by a comma. But not with a copula. English follows the common Germanic habit in that case and exposes its long forgotten V2 nature.
So what is that there? It's an adverb. No one would argue it was a "prepositional subject" because there's no preposition. It's an adverbial. It tells the place. It is not the subject. The subject is of course The best place.
And it's the same for the original example sentence.
The best time would be from five to seven.
From five to seven would be the best time.
Again, those sentences are equivalent but for the topic. From five to seven is not the subject. It's an adverbial that is coupled to the subject The best time.
And it's the same in German.
There are no prepositional subjects.
I see, further down she cites The Cambridge Grammar of the English Language with the sentence
Over a year was spent on this problem.
and indeed over a year is the subject in this sentence. The problem with her argument however is that this over isn't a preposition. But an adverb that describes a year in more detail. It's more than a year.
It's not a direction we are talking about, and neither does it mean during —that would both be the preposition over— and neither it's the prepositional object of the phrasal verb to spend on — that one is on this problem.
This is very different from her own initial example
Over the mantle is a good place for the mirror.
where over is indeed a preposition as it describes a direction, and of course over the mantle isn't the subject in this case. But an adverbial that is connected to the subject a good place for the mirror.
